Separator for sealed lead-acid battery

ABSTRACT

A sealed lead-acid battery includes a separator made from hydrophilic treated sheet made from synthetic fiber which wraps at least either one of a positive electrode and a negative electrode. A mat type separator made from fine glass fiber is disposed between the wrapped electrode and its opposite electrode. This structure is adopted in a battery which uses a paste type electrode where an expanded grid having no outer frame on both edges is used, thereby prolonging cycle life even after cycling of charge and deep discharge.

FIELD OF THE INVENTION

The present invention relates to a sealed lead-acid battery, and moreparticularly to a separator thereof which prevents an internalshort-circuit and thereby realizes a longer-life battery.

BACKGROUND OF THE INVENTION

In general, a discharge reaction of the lead-acid battery produces leadsulfate (PbSO₄) through electrochemical reduction and oxidation of leaddioxide(PbO₂) and lead(Pb) in sulfuric acid (H₂SO₄) electrolytesolution; where PbO₂ and Pb are active materials of positive andnegative electrodes. The produced PbSO₄ on the positive and negativeelectrodes is electrochemically reduced as well as oxidized by charging,thereby not only producing PbO₂ and Pb, but also revitalizing H₂SO₄. Theoverall reaction of charge/discharge of the lead-acid battery isdescribed as the following formula (1):

The following characteristics are desirable for the separator of thelead-acid battery:

1. Acid-resistant, anti-oxidation and anti-reduction:

Since strong-acid H₂SO₄ may be used as electrolyte, and also,electrochemical oxidation as well as reduction is repeated on both thepositive and negative electrodes through charging and dischargingprocesses, a chemically stable material from which a harmful substanceis not soluble should be selected.

2. High ion-conductivity and no possibility of internal short-circuitdue to contact between the positive and negative electrodes:

Electrolyte is easy to permeate and diffuse through the separatordisposed between the positive and negative electrodes because hydrogenion H⁺and sulfate ion SO₄ ⁻² are of electrolytic dissociation in theelectrolyte together with active materials PbO₂ and Pb of the positiveand negative electrodes and are reaction species, whereby the separatoris vulnerable to be permeated as well as diffused with the electrolyte.In order to avoid contact between the positive and negative electrodes,it is desirable for the separator to be long and bent in shape, and tohave a micro-porous construction. Particles in the active materials ofthe positive electrode, by charge/discharge cycling, tend to drop froman electrode grid which is an electric collector and active materialholder. Thus caution is desirable to avoid this particle drop. To bemore specific, a micro-porous sheet made of rubber or resin, and pulp orglass-fiber reinforced with resin, is used as the separator.

A paste type electrode plate process which is in high productivity iswidely used, among others, in the lead-acid batteries. When using thistype, a synthetic resin sheet having the micro-pore structure and aglass mat are used for the separator. The glass mat is in contact withboth sides of the positive electrode, thereby preventing the activematerial of the positive electrode from dropping. As a result, thenumber of internal short-circuits is reduced. This example issubstantially effective when the electrode plate is used in a ventedtype car battery which is exposed to violent external forces such asvibration, shock, and acceleration.

Recently, Japanese utility model H07-34555 discloses another idea asfollows which further reduces the internal short-circuit. An envelopetype separator made from micro-porous synthetic resin sheet includes aglass mat which is broader than the separator, being laid on theseparator, wherein the separator is folded so that side portions of thesynthetic resin sheet may contact each other. Both edges where thesynthetic resin sheet side portions are contacting and the outer edgesof the glass mat adhere in order to form an envelope.

This separator is also developed to be used in a vented type battery,where the negative electrode is accommodated into the envelope, and thepositive electrode is disposed to contact an outer surface of the glassmat. The electrode group thus formed is incorporated in a cellcontainer, thereby forming the cell.

In order to make maintenance of the lead-acid battery easy, a sealedlead-acid battery has been widely used. Oxygen (O₂) gas generated fromthe positive electrode by over-charging is eliminated by an oxygen cyclereaction on the negative electrode. The same process is seen also in asealed nickel-cadmium battery.

The separator of the sealed lead-acid battery desirably has thefollowing functions other than the above:

(a) O₂ gas, generated from the positive electrode during a periodbetween an end of charging and over-charging of the positive electrode,flows to the negative electrode with ease because of excellentventilation, and the electrolyte is prevented from being fluid.

(b) The electrolyte pertinent to charge/discharge is retained as much aspossible around the positive and negative electrodes.

In order to turn the electrolyte to a non-fluid condition, two methodsare available:

(1) Retainer method: The electrolyte is absorbed into the positive andnegative electrodes as well as the separator so that the electrolyteturns into the pore of solid substance.

(2) Gelled method: The electrolyte turns into gell with colloidalsilica.

The retainer method is now mostly used, and it is sometimes called“Absorbed method” or “Starve method.” The retainer method adjusts aquantity of the electrolyte so that ventilation works in some part whenthe electrolyte is absorbed into the mat type separator and turned intosolid. A mat sheet made from fine glass fiber is widely used as aseparator satisfying the above functions. This separator made from themat type glass fiber is a kind of non-woven cloth that can include shortfibers made of borosilicate glass having a diameter of not more than 1μm.

FIG. 3 shows a typical structure of an electrode group used in aconventional sealed lead-acid battery. Mat type glass fiber separator 3is folded to form a U-shape, and negative electrode 1 is insertedtherein, then the outer side of the folded separator 3 is nipped bypositive electrode 2, and whereby the electrode group is formed.

When using electrodes of the same dimension, a number of plates isincreased in step with capacity, and the electrode group is formed inthe same manner. In general, the number of negative electrodes 1 islarger than that of positive electrodes 2 by one. The electrode group isaccommodated into the cell container, and the most appropriate quantityof electrolyte is poured therein. Finally, the cell container and itscover are sealed to complete the cell.

In this case, the separator 3 is shaped longer than both sides and upperends of the negative electrode 1 and positive electrode 2 in order toavoid an internal short-circuit. This structure finds no problem whenused in a battery of small size, relatively low capacity, and thepositive and negative electrodes with a casting grid having an outerframe; however, when used in a car battery or an electric vehicle (EV)battery, they are relatively high capacity, and in a sealed lead-acidbattery, internal short-circuits sometimes occur, because thesebatteries are exposed to violent outer forces such as vibration, shock,and acceleration. When an expanded grid which has no outer frame on bothedges is used in the electrodes, an internal short-circuit often occursparticularly in cycling of charge and deep discharge, because the activematerials expand in step with increasing cycles of charge/discharge,which expands an edge of the positive electrode to overgrow the edge ofthe separator, and whereby the positive electrode makes contact with thenegative electrode.

The envelope type separator of which both sides are sealed has been thusproposed in order to solve the above problem; however, it has beendifficult to tie both edges of the separator comprising a mat type sheetmade of glass fiber while keeping high productivity. Thus felt typenon-woven cloth is adopted, which is made from acid proof andthermoplastic synthetic fiber such as polyester or polypropylene, andfine powder of acid clay as well as fine glass fiber mixed therein.Namely, the felt-type non-woven cloth of which dimension is large enoughto fold the electrodes therein is doubled-back to form a U-shape, andboth edges thereof are heat-sealed to make the envelope type separator.Then the electrode is inserted into this envelope. An example of thismethod is disclosed in Japanese Patent Laid-open H07-60676, where anenvelope separator is used, which comprises felt-type non-woven clothmade from thermoplastic synthetic fiber such as polypropylene and fineglass fiber evenly dispersed therein. Although this separator has twoadvantages including (a) retaining electrolyte and (b) goodheat-sealing, it has less ability of retaining electrolyte than themat-type separator purely made from glass fiber, and yet, ventilation isnot enough since the diameter of synthetic fiber is in general largerthan that of glass fiber. While increasing the glass fiber content inorder to improve the retainability of electrolyte, tightness ofheat-sealing lowers, i.e. these two factors are in a trade-off relation.Therefore, the separator disclosed in H07-60676 does not work well in asealed lead-acid battery with the glass fiber content of 10-25 wt %defined in H07-60676.

SUMMARY OF THE INVENTION

A separator made from hydrophilic treated synthetic fiber surrounds atleast one of a positive and a negative electrode, and another separatormade from fine glass fiber is also used. This structure results in thefollowing advantages:

(a) Electrolyte between the positive and negative electrodes does notdry up.

(b) The separator made from hydrophilic treated synthetic fiber nonwoven and thermoplastic cloth can be shaped into an envelope or a tubeby a heat-sealing method. Therefore, even when charge and deep dischargeis cycled in a battery using an expanded grid which does not have anouter frame, an internal short-circuit is prevented. Thus, the presentinvention can prolong the life of a sealed lead-acid battery of whichcapacity is comparably large, such as a car battery and an EV battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-1(b) are perspective views and FIG. 1(c) is a side view of anelectrode group used in the sealed lead-acid battery, respectively,according to Exemplary Embodiments 1-3 of the present invention.

FIG. 2 compares the cycle lives between a conventional sealed lead-acidbattery and a battery according to the present invention.

FIG. 3 is a perspective view of an electrode group used in aconventional sealed lead-acid battery.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is now detailed by referring to the drawings andtables.

Exemplary Embodiment 1

FIG. 1(a) is a perspective view of an electrode group used in the sealedlead-acid battery according to the present invention. In FIG. 1(a),negative electrode 1 is located within separator 4 and is shaped like anenvelope. The separator 4 may be hydrophilic treated non-woven clothmade from a mixture of (a) polyethylene(PE) fiber and (b) polypropylene(PP) fiber. Two pieces of separator 3 (which may be a mat sheet in whichthe fibers run in random directions) made from fine glass fiber having adiameter of about 0.8 μm are in contact with both sides of the envelope,and positive electrode 2 is in further contact with the outside thereof,thereby forming an electrode group. The negative electrode 1 and thepositive electrode 2 are electrode plates (for example, paste type)using an expanded grid made from a Pb-Ca group alloy. The non-wovencloth mixed with PE and PP is dipped in fuming sulfuric acid, therebysulfonating the surface thereof, thus the hydrophilic treatment isprovided to the non-woven cloth. After this treatment, the non-wovencloth is washed with water and is dried. Then the non-woven cloth mixedwith PE and PP is doubled back to form a “U” shape, and facing edges ofboth faces are heat-sealed to form an envelope. An exemplary separatormade from hydrophilic treated non-woven cloth has a pore size of 4-27 μmand 0.2 mm in thickness of the cloth, while the mat type separator madefrom glass fiber is 1.0 mm in thickness. These thickness values aremeasured when a pressure of 20 kg/dm² is applied.

Negative electrode 1, positive electrode 2, envelope separator 4 madefrom hydrophilic treated synthetic fiber non-woven cloth, and mat typeseparator 3 made from glass fiber are used to produce an exemplarysealed lead-acid battery A in accordance with the present invention, ofwhich the nominal capacity is 60 Ah at a 3 hour-rate, and the nominalvoltage is 12V having a mono-block with 6 cells.

For a comparison purpose, the following two conventional type sealedlead-acid batteries B and C are produced.

Battery B comprises a mat-type separator made from glass fiber, of whichthe thickness is 1.2 mm, and

Battery C comprises micro porous PE sheet of which the average pore sizeis 3.0 μm and the thickness is 0.5 mm. The mat type separator has athickness of 0.7 mm. Materials other than the separator of batteries Band C are the same as those of battery A. The structure of the threebatteries mentioned above are listed in Table 1.

TABLE 1 Battery Separator Type Type Thickness* A of exemplaryhydrophilic treated PE, PP non-woven 0.2 mm Embodiment 1 cloth (envelopetype), 1.0 mm mat glass fiber B of conventional mat glass fiber 1.2 mmtype 1 C of conventional micro porous PE sheet (envelope type) 0.5 mmtype 2 mat glass fiber 0.7 mm *The values are measured at 20 kg/dm²

A specified quantity of H₂SO₄ electrolyte is poured into the abovebatteries at ordinary temperature, and the battery container is sealedafter receiving an initial charge (formation.) Then, each battery isdischarged with 1/3 C (20A) constant current at 25° C. down to 9.9V.After each battery is charged at 25° C., each battery is dischargedagain with 2.5C (150A) constant current. The result of this initialcapacity test is shown in Table 2. The discharged capacity is measuredat a relative value based on Battery B=100.

TABLE 2 Battery type Discharge with 1/3C Discharge with 2.5C A 100 100 B100 100 C Not discharged Not discharged

Regarding “charging”, a two-step constant current charging method isadopted, i.e. charging each battery with 1/5C (12A) constant currentuntil a terminal voltage reaches 14.4V, then switching the constantcurrent to 1/20C (3A) and charging each battery again for four hours.

Table 2 proves that the battery A of the present invention has aperformance at a low-rate (1/3C) discharge and a high-rate (2.5C)discharge like that of the battery B having a separator made from matglass fiber and featuring an excellent discharge performance.

On the other hand, the battery C adopting the envelope type separatortogether with the mat glass fiber separator cannot discharge even at alow-rate (1/3C) after formation in the battery container, and thus datais not measured. The battery C cannot discharge at the high-rate (2.5C)either. The reason may be insufficient diffusion of electrolyte betweenthe positive and negative electrodes because the electrolyte in theseparator, which is directly in contact with the positive electrode andis made from micro porous PE sheet shaped in the envelope, dries up. Theelectrolyte becomes starved due to sealing the battery, and theelectrolyte is retained by the mat glass fiber separator. The separatorused in the battery C is effective for a vented type battery whichexists with ample electrolyte; however, it is not recommended for thesealed type battery.

The inventors tested the battery A of the present invention and theconventional battery B for cycle life by cycling the above mentionedcharging and discharging at the low-rate(1/3C), where an end voltage is9.9V and a test temperature is 25° C. The test result is shown in FIG.2.

The capacity of the battery A reduces a bit until charge/dischargereaches 500 cycles, and then the capacity is reduced slightly. When 80%of the is initial capacity (when the cycle hits 700) is reached, thetest is ended. In the meantime, battery B, which shows approximately thesame initial performance as battery A, reduces its capacity remarkablyafter 200 cycles, and reaches 80% of the initial capacity at 300 cycles,and finally an internal short-circuit occurs at 350 cycles. The batteryB thus cannot be recharged. The battery B is disassembled to find that agrid of the positive electrode extends and touches the active materialwhich has overgrown from both edges of the positive electrode and thenegative electrode, and the internal short-circuit occurs.

Exemplary Embodiment 2

The same materials as used in Embodiment 1 are used in Embodiment 2,i.e. the envelope type separator comprising the positive and negativeelectrodes and hydrophilic treated sheet (e.g. non-woven cloth) madefrom synthetic fiber, and the separator (e.g. mat type) made from fineglass fiber. As shown in FIG. 1(b), the positive electrode 2, whichdiffers from Embodiment 1, is situated in the envelope type separatorcomprising hydrophilic treated non-woven cloth 4 made from syntheticfiber. The mat type separator 3 made from glass fiber is on both sidesof the envelope type separator, and negative electrode 1 is further incontact with the outside thereof, so that the electrode group can beformed. This electrode group is adopted into an exemplary battery D inaccordance with the present invention. The initial capacity and cyclelife of the battery D are approximately the same as those of the batteryA. The battery D and the battery B of conventional type are tested forpass-through of foreign matter attached to the electrodes by a vibrationtest. The foreign matter, a notched piece of the positive electrode 2having, for example, a size of 0.7 mm, is intentionally left on theelectrodes in forming the electrode group when the batteries areproduced.

The electrolyte is poured, initial charge (formation) is completed, andthe battery containers are sealed. After the initial capacity test iscompleted, the batteries are charged, and the following vibration testis applied to these batteries D and B:

Acceleration=3.5G, Sweep speed=1 Hz/sec, Frequency range=10-60 Hz (sweepthis range repeatedly)

In the battery B, an internal short-circuit occurs in approximately 100hours when the test starts. The battery B is disassembled to find thatthe active material of the positive electrode has passed through the mattype separator made from glass fiber. The active material from thepositive electrode permeates into the mat type separator whose thicknessis 1.2 mm by its half thickness in depth and in almost the entire area,and the internal short-circuit is found where a notched piece of activematerial is intentionally left on the positive electrode. On the otherhand, the internal short-circuit does not occur in 400 hours afterstarting the vibration test in the battery D. The battery D isdisassembled when 400 hours have passed to find that there is littledamage due to the permeation of the active material into the mat typeseparator. The envelope type separator comprising hydrophilic treatednon-woven synthetic fiber may prevent the permeating of active materialfrom the positive electrode.

The above embodiment proves that the battery, where the envelope typeseparator comprises hydrophilic treated non-woven synthetic fiber thatsurrounds the positive or negative electrode therein, and where the mattype separator made from glass fiber is disposed between the oppositeelectrode and the envelope type separator, can not only maintain thesame discharging performance as the conventional battery which usespurely the mat type separator made from glass fiber, but also realizes areduction of internal short-circuits, and thereby the cycle life of thebattery is dramatically prolonged.

In Embodiments 1 and 2, either one of the positive or negative electrodeis accommodated into the envelope type separator made from hydrophilictreated non-woven cloth made from synthetic fiber; however, if possible,both of the positive and negative electrodes may be accommodated intothe above envelope type separator, and the mat type separator made fromglass fiber may be disposed between these two electrodes. This structurecan further prolong the cycle life.

Exemplary Embodiment 3

There is another method to obtain a similar effect as in Embodiments 1and 2: As shown in FIG 1(c), two electrodes (shown as positiveelectrodes 2) or two electrodes having mat type separators made fromglass fiber (not shown) are inserted into a tubular separator comprisinghydrophilic treated non-woven cloth 4 made from synthetic fiber so thateach lower end of the two electrodes may be adjacent, then the tubularseparator is doubled back to form a “U” shape. Next, the opposite polarelectrode to those in the tubular separator (shown as negative electrode1) is disposed to contact with the inside and outside of the “U” shape.The opposite polar electrode disposed to contact the “U” shaped tubularseparator can have a mat type separator 3 made from glass fiber whichcontacts the “U” shaped tubular separator, as shown in FIG. 1(c). Themat-type separator 3 can comprise a piece positioned adjacent to each ofthe facing surfaces of the negative electrode 1, as shown in FIG. 1(c).Alternatively, the mat-type separator can be formed from one piece thatis folded into position adjacent to the facing surfaces of the negativeelectrode. An electrode group so structured can produced the same effectas Embodiments 1 or 2.

In the above embodiments, a sulfonating method is adopted forhydrophilic treatment; however, other methods such as graftpolymerization, oxidation, and plasma irradiation are available, and asfar as those methods can change the surface property of synthetic fiberto a hydrophilic property, those methods also can be adopted.

In the above embodiments, the non-woven cloth made from synthetic fiberis used, and the cloth is produced by mixing PE and PP. The materialused in this embodiment is not limited to the non-woven cloth, but wovencloth can be also used. Other synthetic fiber materials such aspolyvinyl chloride, polyethylene terephthalate, polyethylene,polypropylene, or a mixed product of those materials can be used, as faras the materials satisfy those properties such as being acid-proof,withstanding the electrochemical oxidation and reduction, and preferablythermoplastic. The hydrophilic treated and thermoplastic cloth made fromsynthetic fiber can be formed into the envelope type separator with easeby doubling-back the cloth to form a “U” shape and then heat-sealing thefacing edges, and also can be formed into the tubular separator withease by doubling-back the cloth to form a “U” shape and heat-sealing theopposite ends to the fold. The thickness of the above cloth is 0.1-0.7mm, and the mat type separator made from glass fiber can be mosteffectively used provided its thickness ranges from 2.0 to 0.5 mm.

As detailed in the above embodiments, the positive or negativeelectrodes are wrapped in the envelope type separator comprising thehydrophilic treated cloth made from synthetic fiber which is usedtogether with the mat type separator made from glass fiber, therebyreducing the change of an internal short-circuit, and as a result, thecycle life of the sealed lead-acid battery can be prolonged. In thiscase, a paste type electrode using not only a cast grid with an outerframe, but also using the expanded grid without an outer frame on bothedges, is adopted for the positive and negative electrodes, and thebattery is exposed to violent forces such as vibration, shock,acceleration, and charge and deep discharge.

The present invention dramatically improves the performance of thesealed lead-acid battery, and reduces maintenance of car batteries aswell as EV batteries.

What is claimed is:
 1. A sealed lead-acid battery comprising anelectrode group which comprises: (a) a tubular separator made from ahydrophilic treated cloth made from a synthetic fiber, (b) twoelectrodes of the same polarity inserted into said tubular separator sothat lower ends thereof become adjacent to one another, (c) glass fiberseparator portions in contact with an outside surface of said tubularseparator, and (d) an electrode opposite in polarity to said twoelectrodes having one of said glass fiber separator portions on eachside thereof upon folding of said tubular separator.
 2. A sealedlead-acid battery comprising an electrode group which comprises: (a) atubular separator made from hydrophilic treated cloth made fromsynthetic fiber, (b) two electrodes of the same polarity inserted intosaid tubular separator so that lower ends thereof become adjacent to oneanother, (c) at least one glass fiber separator made from fine glassfibers, said at least one glass fiber separator being in contact with anoutside surface of said tubular separator, wherein said at least oneglass fiber separator is one of a) a doubled-back “U” shape member andb) two glass fiber separators, and (d) an electrode opposite in polarityto said two electrodes having said glass fiber separator on each sidethereof upon folding of said tubular separator.
 3. A sealed lead-acidbattery comprising an electrode group which comprises: (a) a tubularseparator made from hydrophilic treated cloth made from synthetic fiber,(b) two electrodes of the same polarity inserted into said tubularseparator so that lower ends thereof become adjacent, (c) at least oneseparator made from fine glass fibers either doubled-back to form a “U”shape or provided in two pieces, said two electrodes being in contactwith the inside surface of said tubular separator, and (d) an electrodeopposite in polarity to said two electrodes having said glass fiberseparator on each side thereof upon folding of said tubular separator.4. A sealed lead-acid battery comprising: a tubular separator; twoelectrodes of the same polarity inserted into said tubular separator sothat lower ends of said electrodes are proximal to one another; anopposite electrode; and a separator adjacent to facing surfaces of saidopposite electrode; said tubular separator being folded to at leastpartially surround said opposite electrode and said separator.
 5. Thesealed lead-acid battery as recited in claim 4, wherein said separatoris folded into position adjacent to said facing surfaces of saidopposite electrode.
 6. The sealed lead-acid battery as recited in claim4, wherein said separator comprises a piece positioned adjacent to eachof said facing surfaces of said opposite electrode.